A Classic Tube Tester

Funny how things come full circle. When I was a young electronics experimenter, tube equipment was cheap and solid state devices were expensive and exotic. I could work with low-power diodes and transistors, but to accomplish anything with some power, tube circuits were king. Parts were salvaged from old tube equipment, and surplus radios and transmitters were all tube powered. My grandfather had learned how to service radio and television equipment in the 1950’s after he retired from farming, and by the 60’s he was done with that too. I inherited his test equipment which included a tube tester — but more on that later.

The 1970’s saw solid state power devices came of age, and we all wanted solid state stereo and ham equipment. Digital electronics were new and fascinating and the old tube equipment just seemed outdated, cumbersome and heavy. Boat-anchors. By the time I moved to Texas in the 80’s to go to school, I had gotten rid of most of that old stuff including all of my grandfather’s test equipment. It was just too big and heavy to move to Austin.

Fast forward to now, and I seem to be collecting classic old equipment. Tube amplifiers are now the audiophile’s choice, and I’ve been interested in pre-war shortwave radios for years. Acting on that impulse almost without knowing it, I’ve accumulated about a dozen classic old receivers — almost all of which use tube technology. In order to restore these old beauties, one must (among other things) test some tubes. Sure would be nice to have that old tube tester again.

So I’ve been looking at flea markets, hamfests and eBay, but it seems everyone has the same idea and tube testers are surprisingly expensive. I finally had a little luck a few weeks ago, and was able to purchase a Precision Model 612  for just over $100 on eBay. It’s condition was unknown, so it was a bit of a risk, but I figured it was worth a gamble. I just hoped that the power transformer or some other crucial but unobtainable part wasn’t toasted.

It arrived a few days later in a large box filled with styrofoam peanuts. As I pulled the wooden cabinet out of the box and opened it on the bench, it felt very familiar. It took me a few minutes, but then it hit me — it was the same model of tube tester I had left behind in Omaha over 30 years ago! How weird it that?

The rubber-covered power cable was cracked and dangerous, along with the crunchy test leads, and there was some damage to the veneer on the wooden case, but otherwise it looked ok. Against my better judgement, I couldn’t help plugging it in to see if it would light up. No soap. He’s dead Jim. A schematic and manual weren’t hard to find on the Web, and it was out of the case in no time.

A little testing revealed the problem: the 300-ohm wire-wound resistor that adjusted the line voltage was open. It took about an hour of shopping on the Web to find a replacement, and to my surprise, Amazon had a Chinese part that would substitute just fine. Not as good as an American part, but at $10 vs. $70, if was worth a chance. Throw in a replacement power cord and I was set.

The following weekend, I went to work.  Removing the old rheostat wasn’t hard, but the replacement part presented a challenge. Although very close in size, it was made to metric standards and a little finagling was needed to get it mounted. After soldering connections, I replaced the disintegrating AC power cord. We should be in business.

Sure enough, the power light came on and the meter deflected to near the “set” mark. The rheostat adjustment worked fine, and I now knew that the most exotic parts, the power transformer and meter were ok. Pulling a tube out of one the many receivers in my shop, I searched the settings chart. Settings made, I plugged the tube in and waited for it to heat up. After what seemed like a reasonable time, I pressed the “test” button and the meter deflected into the green “good” zone. It worked! But wait, the “short” light had come on. The tube was shorted? I tried another. It too was shorted. Bad luck, but it could happen. How about a third? Same result. Ok, this it getting a little strange. I ended up going through about a dozen tubes and the all had the same problem: shorts. Coincidence? Probably not. There was something still wrong with the tube tester. Back to the Web.

In a relatively short time, I came across a YouTube video showing the same problem as well as a sluggish meter movement. I had also observed that the meter seemed to “hang” at about 75% scale.  Both fixes were pretty simple: replace the one capacitor in the tester to fix the short problem, and take the meter apart and check for corrosion on the meter movement permanent magnets.

Getting the meter out of the tester was a little tricky, but I finally got it. After disassembling the meter I didn’t find any corrosion per se, but I carefully cleaned the space between the moving coil and the horseshoe magnet anyway. Replacing the old .1 MFD wax/foil capacitor with a new “orange drop” cap was much easier. Everything back together and time another test.

This time everything worked just fine. I retested all the “shorted” tubes, and this time they were fine. The meter movement, although still a little sluggish above 75%, was much improved. I have to thank K5CXO for his helpful video on repairing these two problems. And now, I’m ready to test all the tubes in my current and future restoration projects. Gramps, the circle is unbroken, and I hope you’re somewhere smiling about this very good fortune!